Trunked radio communications systems are commonly used to provide radio communications between a plurality of mobile terminals and one or more base stations using a plurality of control and traffic channels. In general, a radio communication is set up between a mobile terminal and the base station, or between two mobile stations using a control channel. For example, a mobile terminal can send a communications request over a control channel requesting the use of a traffic channel. Depending on the current system usage, an available traffic channel can be allocated to the mobile terminal by sending a return control message to the mobile terminal over the control channel identifying the allocated traffic channel.
A conventional trunked radio communications system is illustrated in FIG. 1. As shown, the base station includes a controller 25 and a plurality of base station antennas 23 wherein each base station antenna 23 services a different geographic area 36. Moreover, the communications system can be a simulcast system wherein the same radio signals 32 are transmitted simultaneously from each of the base station antennas. Accordingly, once a traffic channel is assigned to a mobile terminal, the mobile terminal can communicate using the traffic channel as it moves between geographic areas 36 without requiring handoffs from one base station antenna to the next.
Simulcast systems are commonly used, for example, by fire departments, police departments, or other government organizations as well as by private organizations requiring a dedicated communications system. In such a system, a mobile terminal 21 transmits a control message over a control channel when radio communications are desired. This control message may be a digital message including an identification number for the requesting mobile terminal, as well as a priority status indicator which indicates a relative priority of the communications request. For example, the priority status indicator can indicate one of a low priority status such as a non-emergency status, or a high priority status such as an emergency status.
This priority status can then be used by the communications system to prioritize the assignment of communications channels. For example, a communication channel can be allocated responsive to a high priority request before a communications channel is allocated responsive to another low priority request, or a low priority communication can be interrupted to provide a communication channel responsive to a high priority request.
Transmission errors such as bit slips, however, may occur reducing the reliability of the communications system. For example, multiple transmission paths between a mobile terminal and the base station may result in multi-path interference. In other words, a control message transmitted by a mobile terminal may be received by two base station antennas at slightly different times, or the control message may be received twice at the same base station as a result of reflections off terrain or man made structures. Errors may also occur as a result of non-system interference, noise, or fading. In addition, transmission errors may occur with respect to messages transmitted over traffic channels.
To reduce transmission errors, error correction and detection codes, such as Bose-Chaudhuri-Hocquenghem (BCH) codes, have been added to digital control messages. Accordingly, the control messages discussed above may have error detection and correction (EDC) codes appended thereto. In particular, an error detection and correction code can be generated for each digital control message to be transmitted, and this error detection and correction code can be transmitted with the control message. The control message and the error detection and correction code are then received at the base station where an error detection and correction algorithm can be used to detect and correct errors in the received control message using the error detection and correction code. Error detection and correction codes can also be used with digital messages transmitted over traffic channels, as well as messages transmitted from the base station to the mobile terminal.
Bit slips, however, may occur during transmission due to interference as discussed above. A bit slip may result in a received control message with an error which is not detected using conventional error detection and correction codes, or a bit slip may result in a detected error which is incorrectly corrected. In either case, an incorrect control message may be received and processed as if it were correct. Simulations have shown that up to 30% of digital control messages transmitted with bit slips using conventional BCH error detection and correction may be incorrectly received and processed. In particular, it has been found that low priority communications request messages may be incorrectly received and processed as high priority communications request messages as a result of bit slips even though BCH error detection and correction codes are used. Accordingly, low priority communications may be given priority treatment unnecessarily interrupting or delaying other communications. Bit slips may also occur on control messages transmitted back to the mobile terminal thereby delaying communications. In addition, bit slips may occur on digital messages transmitted over traffic channels reducing the reliability of communications.
Similar problems may also occur in cellular communications systems wherein adjacent base station antennas use different communications channels thereby allowing a reuse of communications channels across the system. Because adjacent base stations use different communications channels, it may be necessary to provide hand-offs from a communication channel of a first base station antenna to a communication channel of a second base station when a mobile terminal moves from the area covered by the first base station antenna to an area covered by the second base station antenna. While a cellular communications system may reduce interference between adjacent base station antennas, bit slips may still occur as a result of multi-path effects due to terrain, buildings, and other structures. For example, a signal transmitted by a mobile terminal may follow a first direct path to the servicing base station antenna as well as a second reflected path due to a reflection off a building, and the two signal paths may interfere thereby resulting in bit slips.
Accordingly, there continues to exist a need in the art for improved methods and systems for transmitting and receiving radio communications signals.